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The |flattenhtml| property of grd file entries is considered deprecated
and new usages of it are discouraged. Removing unnecessary usages,
both to:
 - discourage copy/paste usage
 - reveal which files actually still rely on it

The files that don't need it were detected by locally modifying Grit,
to compare the file contents before and after flattenhtml is applied.

In this CL only affecting files that don't need flattenhtml=true nor
preprocess=true. Cases where |flattenhtml| can be removed, but
|preprocess| is still needed will be addressed in a follow-up.

Bug: 1152343
Change-Id: Iee0cb3fd9b9da0cc82a04c4fbd2e2e8a69f4cdc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2555879Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831145}